Question 44 OSE02 - Assistant Engineer - OSV

The Correct Answer is A ### Explanation of Option A Option A is correct because it accurately describes the physical characteristics and operation of a common mechanically-driven auxiliary blower (often termed a scavenging blower) used on diesel engines: 1. **Positive Displacement Type:** Auxiliary blowers intended to supply air at low engine loads or supplement the turbocharger (especially common in 2-stroke or certain 4-stroke designs) are typically **Roots blowers**. A Roots blower is a positive displacement machine, meaning it traps a fixed volume of air between the rotating lobes and the casing for every revolution. 2. **Displacement Directly Proportional to Engine Speed:** Since the scavenging blower in this configuration is usually driven mechanically (via gears or belts) directly by the engine crankshaft, its rotational speed is tied directly to the engine speed (RPM). Because the volume of air delivered is calculated by (fixed volume per revolution) $\times$ (revolutions per minute), the total volume (displacement) of air supplied per minute is **directly proportional** to the engine speed. ### Explanation of Incorrect Options * **B) The scavenging blower is a non-positive displacement type, and the actual displacement is directly proportional to engine speed.** * This is incorrect because the supplementary blower (the scavenging blower) is generally a positive displacement device (Roots blower). The turbocharger is a non-positive displacement device (centrifugal compressor), but the question specifically refers to the scavenging blower. * **C) The scavenging blower is a positive displacement type, and the actual displacement is not directly proportional to engine speed.** * While the first part (positive displacement) is correct, the second part is incorrect. As the blower is mechanically linked to the engine, its displacement *is* directly proportional to the engine speed. * **D) The scavenging blower is a non-positive displacement type, and the actual displacement is not directly proportional to engine speed.** * This is incorrect because the scavenging blower is a positive displacement type. Furthermore, if it were a mechanically-driven non-positive device, its output would still be proportional to engine speed, although the efficiency and flow rate would drop rapidly with increasing back pressure.